Homodyne, or direct conversion receivers are known in the art. As the name implies, direct conversion receivers directly convert an incoming radio frequency (RF) signal into its baseband in-phase and quadrature components without any intermediate translation into an intermediate frequency (IF) signal.
Conventional methods of down converting an RF signal to baseband require two conversion steps. The RF signal is first down converted to an intermediate frequency (IF) signal. Then, the IF signal is down converted to a baseband signal. In a mobile telecommunications environment, this can require multiple chips and other circuits including combinations of elements such as a radio frequency receiver (RFR) chip, an intermediate frequency receiver (IFR) chip, a baseband receiver chip, and other associated surrounding chips such as intermediate filters, mixers and amplifiers. Alternatively, this can require a single receiver chip with both RF and IF circuits and a separate IF filter chip. In either topology, all of these RF circuit components are expensive for manufacturers of small, low cost mobile communication devices such as cellular phones, pagers, cordless phones, two-way radios, etc. Therefore, heterodyne receivers are less than ideal for these applications.
Direct conversion receivers work by translating RF signals directly into base band using a local oscillator (LO) having a frequency exactly matched to the frequency of the received RF signal to demodulate the RF signal. This can be performed in either the analog or digital domain. An incoming signal g(t) is received at the RF input of the direct conversion receiver and then passed through a preselect filter and a low-noise amplifier (LNA). The preselect filter is simply a band pass filter designed to pass the desired signal g(t) and to reject spurious out-of-band signals. The bandwidth of the preselect filter is much greater than the bandwidth of the desired signal. Thus, the preselect filter is a coarse filter and may pass unwanted signals in addition to the desired signal.
After passing through the preselect filter, the signal g(t) is split and each split portion is sent through a mixer circuit. In one mixer circuit, the signal g(t) is mixed with a sinusoid generated by the LO and tuned to the same frequency as the carrier frequency. In the other mixer, the signal g(t) is mixed with the same sinusoid but with a phase change of π/2 radians (90 degrees). The two mixers produce the in-phase and quadrature (I and Q) components of the desired signal g(t) centered at the baseband and at twice the carrier frequency. The high frequency components are eliminated by the low pass filters and the in-phase and quadrature signals are finally amplified.
Direct conversion enables the direct conversion of RF signals to baseband signals in a single step—that is without intermediate frequency signals. Thus, direct conversion eliminates the need for the RF to IF conversion step, and thus, the IF filter chip or other IF circuitry. This is a significant cost savings for device manufacturers and the simplified down conversion process also reduces power consumption thereby enhancing the performance of devices containing the receiver circuitry.
Direct conversion can also be used within a heterodyne receiver as the final conversion from an IF frequency directly to baseband. This provides low frequency baseband signals for easier and lower power analog-to-digital conversion and subsequent demodulation.
Despite the benefit of direct conversion over conventional methods of down converting, direct conversion suffers from some problems as well. It is well known in the art that direct conversion receivers suffer from constant voltage or direct current (DC) voltage offsets. Unwanted DC offsets include static DC levels as well as time varying DC levels. DC offsets can arise from the receiver's local oscillator (LO) self-mixing (due to leakage and re-radiation), 2nd order effects of strong in-band interfering signals, circuit mismatch and interferer self-mixing, each of which can vary with gain setting, frequency, fading and temperature. The DC offset can result in loss of or degradation in receiver sensitivity, selectivity, dynamic range, and analog and digital response times. As a result of these effects, if not cancelled, DC offset degrades signal quality, limits dynamic range through ADC saturation, and increases power consumption.
One method of eliminating DC has been through the use of filtering. In practical application, filtering is a less than ideal solution to the DC offset problem. Filtering suffers from slow response times due to narrow bandwidths and possible loss of portions of the desired signal.
Other problems and drawbacks exist with DC offset correction methods in direct conversion receivers.